WO2002079769A2 - Method and device for detecting the concentration of an oxygen-containing compound in an analyte gas - Google Patents

Abstract

The invention relates to a method and to a device for detecting the concentration of an oxygen-containing compound in an analyte gas in an external volume that also contains oxygen. The inventive method utilizes a sensor with a first chamber that communicates with the external volume via a first diffusion barrier, and with a second chamber that communicates with the first chamber via a second diffusion barrier. The device is further provided with means for reducing the oxygen concentration in the first chamber and means for detecting the oxygen concentration in the second chamber. The oxygen concentration in the first chamber is reduced during a first phase to a value at which no change in concentration of the oxygen-containing compound can be observed. In a second phase, the oxygen concentration in the first chamber is further reduced to a value at which a reduction of the concentration of the first oxygen-containing compound can also be observed. The difference of the oxygen concentration detected in the second chamber in the first and second phase serves as a measure for the respective concentration.

Description

Method and device for detecting the concentration of an oxygen-containing compound in a measuring gas

State of the art

The invention relates to the detection of the concentration of a certain oxygen-containing compound in a measuring gas which also contains further oxygen-containing compounds.

The exhaust gas of an internal combustion engine is of particular interest as the measurement gas. The particular oxygen-containing compound whose concentration is to be recorded can be nitrogen oxide. Molecular oxygen, carbon monoxide, carbon dioxide and water are particularly suitable as further oxygen-containing compounds.

From SAE Paper 960334 and from EP 678 740 a NOx sensor with two chambers is already known, which are delimited by an ion-conductive solid electrolyte. The first chamber is above the first diffusion limitation with the

Sample gas in connection. The first and a second chamber are interconnected by a further diffusion limitation. The concentration of oxygen in the first chamber is reduced to a predetermined value by a first electrolytic oxygen pump cell. For this purpose, a pump voltage is applied to the electrolyte via electrodes so that the resulting electric field in the electrolyte drives a flow of oxygen ions out of the chamber. By choosing the right physical conditions like

- the temperature,

- The selective catalytic effect of the electrode material and

- The pump voltage that certain oxygen-containing connection

(for example NOx), the concentration of which is to be recorded, makes no contribution to the oxygen pumping current from the first chamber. If, for example, nitrogen oxides represent the specific oxygen-containing compound, the physical conditions mentioned can be specified in such a way that decomposition into nitrogen and oxygen does not occur at the electrode of the first chamber. Oxygen from this compound can then not be pumped off and thus make no contribution to the oxygen pumping current of the first chamber.

A second electrolytic pump cell in the second chamber further reduces the oxygen concentration. As the oxygen concentration decreases, the nitrogen oxides are separated into oxygen and nitrogen. The strength of the pump current in the second chamber is used as a measure of the NOx concentration in the sample gas. The principle of this sensor is based on the fact that the pumping out of oxygen from the second chamber disturbs the equilibrium of the reactions NO <-> 1 / 2N2 + 1/2 02 and favors the right side of the equation.

Since NO is broken down in the second chamber during the oxygen pumping process, the NOx compounds also feed the Pumping current of the second chamber. In addition, because the 02 concentration is assumed to be known and constant by the upstream first pump cell, the amount of NOx in the measurement gas can in principle be determined by the strength of the pump current from the second chamber.

State of the art sensors have an offset, a drift and a strong temperature dependence of the pump current. These disadvantages are based on the following causes:

The electronic residual conductivity of oxygen-conducting solid electrolytes as well as Seebeck voltages and electrical field and current coupling create offset sizes in the nitrogen oxide measurement signal even in the absence of nitrogen oxides. As a result of structural changes during operation, the electrical properties of the solid electrolyte change in such a way that a drift occurs both in the signal amplitude and in the signal zero position.

Against this background, the object of the invention is to minimize the offset, the age drift and the temperature dependence of the signal for the desired concentration of the specific oxygen-containing compound.

The desired effects are achieved by the features of the independent claims.

In particular, in a method for determining the concentration of an oxygen-containing compound in a measuring gas in an external volume, which also contains further oxygen, a sensor with a first chamber is used, which is connected to the external volume via a first diffusion barrier and which is connected to a second chamber is connected via a further diffusion barrier. Next are means to reduce the

Oxygen concentration in the first chamber and means for detecting the concentration of oxygen in the second chamber are provided. According to the invention, the oxygen concentration in the first chamber is reduced in a first phase to a value at which a change in the concentration of the oxygen-containing compound does not occur, and further in a second phase there is a further reduction in the first chamber to a value in which a decrease in the concentration of the 1st oxygen-containing compound occurs. The stated measure for the concentration sought is formed as the difference between the oxygen concentrations detected in the 1st and 2nd phase in the second chamber.

In contrast to evaluation methods, which are known from the prior art and in which the voltage of the pump electrode of the first chamber is controlled to a fixed value to the air reference electrode, then a reversal inventively alternately between two values U _IPE _ _H, Uι _PE _ _L ,

The pumping current of the second chamber is recorded both at Uι _PE _ _H and at U _IPE _ _L. The difference between the pump currents of the second chamber at U _IPE _ _H and U _IPE _ _{L is} formed as a measure of the concentration sought for the specific oxygen-containing compound.

The formation according to the invention of the difference between the values of the at two different pump voltages

Pump current from the second chamber causes the desired minimization of the offset, the age drift and the temperature dependence. The difference formation effectively causes a zero adjustment of the pump current of the second Chamber. In other words, the method according to the invention can also be referred to as a pendulum operation of the first oxygen pump electrode with respect to the pump voltage. The pendulum operation according to the invention enables low-offset and low-drift measurement due to a constant zero adjustment.

This has the following advantages:

Temperature changes in the sensor have a similar effect

Signal size and zero position, so that the pendulum process reduces the temperature coefficient of the nitrogen oxide signal.

In the case of signal errors arising from the current and

Field coupling result, the improvement becomes even greater, since the errors occur almost identically in signal size and zero position.

Due to the constant determination of the zero position, the drift of the zero position loses its importance completely.

Pulse-shaped disturbances, e.g. from high-frequency couplings can be greatly reduced by a validity test of the measured values.

Exemplary embodiments of the invention are explained below with reference to the figures.

Fig. 1 shows a double-chamber NOx sensor, as for

Implementation of the inventive method is suitable.

2 shows a circuit for operating the sensor according to the invention. FIG. 3 reveals signals over time as they occur when the method according to the invention is carried out.

The number 1 in FIG. 1 denotes a solid electrolyte with three chambers 2, 3 and 4, diffusion barriers 5 and 6 and electrodes 7 to 11 and a heater 12.

The first chamber 2 is connected to the exhaust gas via the diffusion barrier 5 and to the second chamber 3 via the second diffusion barrier 6. The first or the second chamber can in turn consist of several chambers. The third chamber 4 is only connected to the air via a channel and contains the air reference electrode 10.

The oxygen pump electrode 7 serves as the negative electrode of the oxygen pump of the first chamber.

The electrode 8 serves as the negative electrode of the oxygen pump of the second chamber. As described above, the pump current can serve as a measure of the desired concentration of nitrogen oxides or another oxygen-containing compound under certain conditions.

In addition to the electrode 8, a further pump electrode 9 can be provided in the second chamber, the material of which, for example, favors the decomposition of nitrogen oxides.

The positive counter electrode for the oxygen pumping process is a more positive one in direct comparison

Electrode. This can be the outer pump electrode 11, for example.

At the pump electrode 9, the pump current of the nitrogen be measured.

The pump electrode 9 has the most negative potential. Any other electrode can therefore be used as a counter electrode for this electrode. Depending on the field geometry in the solid electrolyte, these can also be several of the other more positive electrodes.

An insulated heater 12 brings the sensor to the necessary operating temperature.

The sensor shown enables the following functions:

The sample gas passes through the first diffusion barrier 5 into the first chamber 2. The sample gas contains not only molecular matter

Oxygen at least one oxygen-containing component, the concentration of which is to be determined in the measuring gas. Nitrogen oxides are an example of such an oxygen-containing component.

Each of the electrodes 7, 8 and 9 forms an oxygen pump in connection with a more positive electrode and the ion-conducting solid electrolyte lying between the electrodes. Electrode 11 is particularly suitable as a more positive counter electrode.

A potential difference between the electrodes creates an electric field in the electrolyte that drives the oxygen ion pump current.

By selecting a suitable electrode material, for example, the dissociation of oxygen molecules can be preferred over the separation of other oxygen-containing compounds. If, for example, the concentration of nitrogen oxides is to be recorded, it is advantageous if the

Oxygen pump electrode 7 promotes the dissociation of oxygen, but not nitrogen oxide.

In contrast, the NO pump electrode 9 should have a nitrogen oxide decomposing effect in this application.

The oxygen pump in the first chamber reduces this

Oxygen content of the measuring gas in the first chamber by pumping out oxygen ions that originate from molecular oxygen, but not from the at least one oxygen-containing component.

In the case of sensors according to the prior art, the potential of the outer pump electrode is regulated to a fixed value in relation to the air reference electrode. The higher the potential difference, the lower the oxygen concentration that arises in the first chamber. The fixed value in the sensors according to the prior art is predetermined such that the oxygen content remaining in the first chamber does not go to zero. This prevents decomposition of nitrogen oxides in the first chamber.

In contrast, the invention uses at least two potential values for the outer pump electrode, between which the sensor is switched during operation.

The sample gas with reduced oxygen content enters the second chamber via a second diffusion barrier. There, further oxygen is pumped out, so that when the oxygen content in the second chamber drops, the at least an oxygen-containing component is activated as the source supplying the pump current. The resulting pump current is recorded as a measure of the desired concentration of the specific oxygen-containing compound.

For the operation of the sensor according to the invention, an evaluation circuit is required which provides different electrical voltages and which obtains the measurement signal from a current measurement.

The block diagram of an evaluation circuit is shown in FIG. 2.

The oxygen pump electrode 7 in the first chamber lies to a certain extent on a virtual ground. This is formed by a current source 2.14, which has a very low internal resistance. The current source 2.14 replaces the negative charge that flows from the pump electrode 7 through the oxygen ion line through the solid electrolyte to the positive electrode 11. The pumping current of the first chamber measured by the current measuring device 2.15 increases with increasing oxygen concentration in the external volume. It can therefore be used as a measure of the oxygen concentration and can thus be used, for example, to regulate the fuel / air ratio for the operation of a combustion process.

The counter electrode 11 is raised to a potential which is positive with respect to ground, whereby it is shifted by the potential of the air reference electrode 10 via the link 2.10. The inversion of the negative provided potential (U_IPE-soll - Potential der

Air reference electrode) in block 2.11 causes within the scope of the illustrated exemplary embodiment that the outer Oxygen pump electrode 11 is positive in comparison to the oxygen pump electrode 7.

The potential U_IPE_soll is generated via a voltage reference 2.1 in conjunction with an operational amplifier 2.2 and set via a controller 2.13.

The voltages for the electrodes 8 and 9 lying in the chamber 2 are generated via voltage references 2.4 and 2.6 and operational amplifiers 2.5 and 2.7. U_02_Soll <zero denotes the voltage reference for the oxygen pump electrode 8 and U_N0_Soll <zero denotes the voltage reference for the NO pump electrode 10.

The potentials of the electrodes 8 and 9 are also shifted by the links 2.8 and 2.9 by the potential of the air reference.

The other electrode potentials are set directly. The NO pump current can be measured using known voltage conversion. This function is represented by the current measuring device 2.12.

According to the invention, U_IPE-soll is not about constant, but is reversed between two voltage _values U _IPE _ _H and U _ιPE _ _L. The control is carried out by a control chip 2.16, which has at least one processor and a program and data memory. A possible time course of U_IPE-Soll in the case of a reversal to be regarded as an exemplary embodiment is shown in FIG. 3a. 3 b shows the corresponding course of the pump current of the second chamber as it is detected by the current measuring device 2.12 and passed on to the control chip 2.16. According to an exemplary embodiment this signal is referred to in FIG. 3b as the NOx signal. If this term is used in the following, it is, as it were, representative of any other oxygen-containing compounds which can be distinguished from other oxygen-containing compounds by measurement technology by the multi-chamber principle described.

The lower voltage U _{IPE L} in FIG. 3 a corresponds to the voltage which is known from the prior art for the regulated state of the first oxygen pump electrode.

This voltage is characterized, among other things, by the fact that the oxygen concentration in the first chamber is reduced on the one hand, but on the other hand a residual oxygen concentration is maintained at which NOx does not yet decompose.

The nitrogen oxides therefore continue to diffuse into the second chamber, are decomposed there and thereby cause the NOx signal with level 1 in FIG. 3b.

The oxygen pump electrodes of the first chamber can pump out oxygen from H ₂ 0, CO and NO and from N0 ₂ at a higher voltage in addition to oxygen from 0 ₂ .

U _ιPE _ _H is now chosen so high that the pump electrode of the first chamber, in addition to the oxygen in the exhaust gas, also pumps out the oxygen which is bound in the nitrogen oxides, for example 500 to 1000 mV, but especially 600 to 850 mV. The nitrogen oxide content in the first chamber then goes to zero, so that the subsequent delivery of nitrogen oxides from the 1st chamber to the 2nd chamber does not occur.

Since oxygen is also pumped out of nitrogen oxides for the measurement in the 2nd chamber, the nitrogen oxide concentration in the 2nd chamber also drops to zero due to the lack of subsequent deliveries. As a result, after the nitrogen oxide concentration has settled in the second chamber against the value zero, the zero value of the signal can be determined. This is the level labeled 2 in Fig. 3b.

The pump voltage acting on the first chamber is then switched to the lower value U _{IPE L.}

After the signal has settled at the lower voltage, the signal value (level 1) with zero value can be recorded.

The subtraction of the signal value with zero value and the previous zero value in control chip 2.16 results in the unadulterated signal value to be output, with which, for example, manipulated variables are formed for the operation of an internal combustion engine. This use of the NOx signal formed according to the invention is represented by block 2.17.

Another exemplary embodiment discloses a possibility of greatly reducing pulse-shaped disturbances in the NOx signal, which originate from high-frequency couplings.

This exemplary embodiment effectively represents a validity test for the sampled signal values. It is explained with reference to FIG. 3. Only the Signal values considered valid that do not have a high rate of change. As a result, in particular the impulsive reactions of the NOx signal to a change in level of the pump voltage are masked out.

Let t = 0 the time of the first change from U _IPE _ _H to U _IPE _ (see Fig. 3a). After the time t _H , the downshift from Uι _PE _ _L to U _IPE _ _H and back to t _{L takes place} again. After the time t _H ι> t _H , a first sample is taken from the nitrogen oxide signal of the evaluation circuit. 3c is represented by the pulses. Others can follow after t _H2 , t _H3 , ... t _Hn . When taking several samples, the difference between two successive samples at time t _H _ and t _H _.ι can be used as a measure for assessing the transient response. If the difference is U _H , U _L; 3d, small enough, the sample values are assumed to be valid. The top line in Fig. 3d shows the sample-hold waveform for the phases with low pump voltage. It corresponds to the NOx signal with the zero value. The bottom line corresponds to the sample and hold waveform for the high pump voltage phases that provide the zero value of the NOx signal.

Signal disturbances that bring about sudden changes are greatly reduced by this validity test.

The final zero value can be formed from the valid values by averaging.

The same procedure is used for the time period t _H <t <t _L , except that instead of the zero values at times t _Lm > t _m -ι ...> t _L2 > t _L ι> t the signal quantities shifted by the zero values are obtained. The final zero value can also be formed from the valid signal sizes by averaging.

The subtraction of the signal value with zero value and the previous zero value results in the unadulterated signal value to be output.

In addition to averaging from the valid signal sizes (zero values), other mathematical functions are also conceivable that can generate a final signal size (zero value), e.g. weighted sums.

Claims

Expectations 1. Method for determining the concentration of an oxygen-containing compound in a measuring gas in an external volume, which also contains further oxygen, with a sensor having a first chamber (2), which is connected to the external volume via a first diffusion barrier (5), and with a second chamber (3), which is connected to the first chamber via a further diffusion barrier (6), with means (7, 11, 2.1, 2.2, 2.10, 2.11, 2.13) for Decrease in oxygen concentration in the first Chamber and with means (2.12, 2.16) for detecting the concentration of oxygen in the second chamber, characterized in that the reduction of the oxygen concentration in the first chamber takes place in a first phase to a value in which a change in the concentration of the oxygen-containing compound does not occur and that in a second phase in the first chamber there is a further reduction to a value at which the concentration of the first oxygen-containing compound also decreases and wherein said measure for the concentration sought is formed as the difference between the oxygen concentrations detected in the 1st and 2nd phase in the second chamber. 2. The method according to claim 1, characterized in that the means for reducing the oxygen concentration in the first chamber comprise a solid electrolyte which is arranged between 2 electrodes to which a voltage is applied. 3. The method according to claim 2, characterized in that the voltage applied to said electrodes in the second phase is higher than in the first phase. 4. The method according to any one of the preceding claims, characterized in that the means for detecting the concentration of oxygen in the second chamber contain a gas generating electrode for generating oxidizable gases. 5. The method according to any one of claims 1 to 3, characterized in that the means for detecting the Concentration of oxygen in the second chamber contain two electrodes (8, 9), one (8) of which decomposes oxygen and the other (9) of which decomposes nitrogen oxides. 6. The method according to any one of the preceding claims, characterized by the exhaust gas of a combustion process as a measuring gas. 7. The method according to any one of the preceding claims, characterized by nitrogen oxides as certain oxygen-containing compound whose concentration is to be determined. 8. Device for determining the concentration of an oxygen-containing compound in a measuring gas in an external volume, which also contains further oxygen, with a sensor having a first chamber (2) which is connected to the external volume via a first diffusion barrier (5) , and with a second chamber (3), which is connected to the first chamber via a further diffusion barrier (6), with means (7, 11, 2.1, 2.2, 2.10, 2.11, 2.13) for reducing the oxygen concentration in the first chamber and means (2.12, 2.16) for detecting the concentration of oxygen in the second chamber, characterized in that the means for reducing the oxygen concentration in the first chamber reduce the oxygen concentration in a first phase to a value at which a change in the concentration the oxygen-containing compound does not occur and that it is the In a second phase, further reduce the oxygen concentration in the first chamber to a value which also results in a reduction in the concentration of the first oxygen-containing compound and means (2.16) which give the measure given for the concentration sought as the difference between that in the 1st and 2nd phase form the oxygen concentrations recorded in the second chamber.